* Validates the submitted user data.
	 *
	 * @since 7.0.0
	 *
	 * @return boolean|WP_Error Returns `true` on success or an error object.
	 */
	protected function validate_user() {

		$validate = llms_validate_user( $this->data );
		if ( is_wp_error( $validate ) ) {
			return $validate;
		}

		// If validation passes, determine if the user already exists and, if they do, validate their enrollment.
		$email = $this->data['email_address'] ?? null;
		$user  = $email ? get_user_by( 'email', $email ) : false;
		if ( $user && llms_is_user_enrolled( $user->ID, $this->plan->get( 'product_id' ) ) ) {
			return $this->error(
				self::E_USER_ENROLLED,
				sprintf(
					// Translators: %s = The title of the course or membership.
					__( 'You already have access to %s.', 'lifterlms' ),
					get_the_title( $this->plan->get( 'product_id' ) )